Optical pattern tracer

ABSTRACT

A rotary scan line/edge tracer using a motor driven mirror to produce the rotary scan. The motor also drives a generator for providing two coordinate reference signals. The mirrors assembly is exchangeable to provide varying scan diameters. The photocell is mounted adjacent the center of the focusing lens at the lower part of the apparatus, thus permitting a coaxial arrangement of the motor and mirror and lens producing a compact assembly. The reference signals are sampled instantaneously to produce coordinate drive signals.

United States Patent Parkeretaly 1 i541 OPTICAL PATTERN TRACER Jewell,Ancaster, Ontario, all of' Canada [73] Assignee: Canadian WestinghouseCompany Limited, Hamilton, Ontario, Canada 22 Filed: Oct. 19,1970 211Appl .No.:81,784

3o Application Priority n w Oct. 20, 1969 Canada ..065212 52 U.S.Cl...250/202, 250/235, 250/236, 350/99 51 Int. Cl. ..G05b1/00 [58]FieldolSearch ..250/202, 203, 234, 2 35, 236; 350/26, 99

[56] References Cited UNITED STATES PATENTS, 3,395,282 7/1968 Blackwell..250/202 1 51 Nov. 28, 1972 6/1963 Wills ..250/202 2,997,598 8/1961Gramrn ..250/236 3,496,437 2/ 1970 Layden ..250/202 3,135,904 6/1964PurIthiser ..250/202 3,017,552 1/ 1962 Brouwer ..250/202 3,004,166 10/1961 Greene ..250/202 3,322,952 5/1967 Jewell ..250/202 Brouwer..250/202 Primary Examiner-James W. Lawrence Assistant Examiner-D. C.Nelms Attorney-R. H. Fox

are sampled instantaneously to produce coordinate drive signals.

2Clains,l2DrawingFigures PATENTEnuuv28 1912 3. 704.372 sum 02 or 12SAMPLE L AND HOLD SPEED PATENTEHNUY28 I972 3,704,372 sum 03 0F 1 POWERSUPPLY NEUTRAL PATENTED Nov 2 8 I972 SHEET on or 1 PATENTEuN0v 28 m2 3.704,372

SHEET 05 HF 1 HIGH SPEED g E TRAVHTRACE q E 5 4 DIRECTION POWER DRIVETRAVERSE PATTERN POWER DRIVE L r J FIG. 3

PATENTEDuuvze I972 SHEET 070F 12 n20 FEE 2O WWW QOP V MOP M i P .Q.%

n? OK ":0 BOP PATENTED NOV 28 I972 SHEET 08 0F 1 v GE Ommmm+ V PATENTEUum 28 m2 SHEET 10 0F 12 M 0.6% or O mI 02655.

PATENTEDNHY 9 3. 704,372

SHEET 11 [1F 12 J 42\ I ,164 F HIGH TRAVERSE STEER SPEED TRACE STRAIGHTCUT FIG. 4f

PATENTEDnuvze I972 PHOTOTRANSISTOR OUTPUT (80) a SIGNAL OUTPUT SINEOUTPUT COSI NE OUTPUT d COMPARATOR OUTPUT (97) SAMPLE PULSE OUTPUT (101)FIG. 5

3.704.372 SHEET 12 [IF 12 +o-o5ov 0 W V T! 1.0V LEVEL(86) THIFEEVSSLOLDII+-- I- LJL iIIA/WMWL OPTICAL PATTERN TRACER This invention relates tooptical pattern tracing controls and in particular, to controls of thetype which repetitively scan the pattern to be followed in a circularmanner, derive a signal from such scanning operation and utilize thesignal to control coordinate drive mtors. I

There are various types of optical pattern tracing machine controlsystems including scanning and nonscanning tracing heads, friction andcoordinate drive machines, edge and line tracers. This invention hasparticular application to a pattern tracer of the circular scanning typefor operation with a coordinate drive system which may be used foreither line or edge trac- By a circular scanning tracer is meant atracer which views the pattern in such a manner that the point observedby the tracing head is caused to rotate repetitively so as to describe'acircular path on the surface bearing the pattern when the head isstationary. Naturally, when the head is in translational motion, thepoint scanned by the tracing head will more closely approach anepicycle.

The detector in the tracing head is arranged to produce a signalindicative of a change in illumination of the detector. In this way, thedevice may operate either as a line tracer, i.e., a tracer of a narrowmark on a surface area, or as an edge tracer, i.e., a tracer of thetransition from a reflective to a less reflective area as in the casewhen the pattern is a silhouette. The signal representing thistransition is then processed and used to control a pair of motors whichwhen associated with suitable machinery, will cause the tracing head andrelated machine tool to more in a plane in accordance with the patternas controlled by the motors.

It will be appreciated that in accordance with general practice in thisfield, the convolutions performed by the tracing head will be similarlyperformed by the machine tool which may, for example, be a cutting torchand in this way, the material to be cut will be shaped into the sameform as the pattern being traced by the tracing head.

In circular scanning tracing heads the diameter of the circle describedby the optically sensitive area as it effectively scans the patternestablishes the lead" of the scanner. Lead, a term in common use in theart, determines the relationship between the steering axis of thetracing head and the point on the pattern being sensed. As will beappreciated, in order for a tracer to respond to deviations in thepattern, information relating to such deviations must be received in thecontrol system before the tracing head has left the pattern, otherwisethe tracer will have left the pattern before compensating action can beintroduced. it will appear, therefore, that the lead of the system mayvary depending on the tracing velocity, the higher the velocity thegreater the lead necessary, assuming other factors in this system remainconstant. It will be seen then, that for any given system a certainminimum lead will be required for a given tracing velocity. On the otherhand, it is well known and understood that as the diameter of thescanning circle becomes larger, the discrimination becomes less and theaccuracy of tracing is reduced. Therefore, for precision, the diameterof the circle must be kept to a minimum.

With these two opposing factors, it will be evident that changes intracing velocity will produce changes in the minimum necessary diameterof the scanning circle and for the ultimate precision, this diametershould be adjustable. In order to produce such adjustable diameter, thetracing head of the present invention provides exchangeable elementswhich vary the scanning diameter.

A machine tool, in most cases actually removes a portion of the materialwhen making a cut. In the case of the gas cutting torch the width of thecut is referred to as the kerf. In order that the piece cut from thematerial shall be the same size as the pattern it is necessary to makecompensation for this kerf; The tracing head of the present invention isarranged to be conveniently adjusted to compensate for removed materialbe producing apparent displacement of the pattern in a direction such asto correct for the kerf in an amount selected at will by the'operator.

In order to provide the desired control a signal is derived from thetracing head and applied to an electronic circuit which accuratelyselects the transition from one condition of reflectivity to anothercondition of reflectivity of the pattern, converts the signal to asuitable form for further processing, and in co-operation with certainother signals from the tracing head which are phase-related to therotational position of the scan, produces a pair of coordinate signals,i.e., an X signal and a Y signal, which are then used to controlcoordinate drive motors.

A clearer understanding of this invention may be had by a considerationof the following specification and drawing, in which:

FIGS. la, lb and 1c together, show a schematic diagram of a tracersystem in accordance with this invention;

FIG. 2 is an elevational view, partially in section, of a tracing headfor use in the system;

FIG. 3 is a view of a control panel showing the location of the variouscontrols for the system;

FIGS. 4A to 4F are schematic diagrams showing the circuit details ofcertain portions dramas masses in the schematic diagram;

FIG. 5 is a series of wave form diagrams useful in an explanation of theoperation of the system.

Considering now the schematic diagram shown In FIGS. la, lb and 1c, itwill be seen that lamps 6 illuminate a drawing 7 and the light from acertain portion of the drawing is focused through the lens 8 andreflected by the mirror 9 onto a photocell 10. The mirror 9 is driven bythe synchronous motor 11, in such a manner that the area of the drawingscanned by the optical system moves in a circular path. Also on theshaft of motor 11 is a timing generator consisting of a permanent magnet12 and a pair of stator windings l3 and 14 arranged physically at rightangles to each other so as to produce two sine waves at right angles toone another or in effect, a sine wave and a cosine wave.

These signals, together with the signal from photocell 10, are appliedto preamplifier 15. The polarity of the sine and cosine wave applied tothe preamplifier is selectably reversable by means of the contacts ofrelay 16. Two outputs from the preamplifier, the first being the signalas amplified, and the other being a signal representative of one halfthe maximum deviation of the signal, are both applied to the logiccircuit 17.

. One output from the logic circuit consists of a pulse indicative ofthe time when the signal traverses a certain level, this timing pulse isused to define the edge of the pattern. Another signal is produced whichis indicative of the presence of a pattern to be followed and thissignal is applied to the on-pattem relay 18 through the contacts of thelight control relay 19.

The pulse output from the logic circuit is applied together with thecosine and sine output from the preamplifier to a sample-and-holdcircuit20. This sample-and-hold circuit produces a pair of signals, onerepresentative of the value of the sine wave at time of occurrence ofthe pulse from the logic circuit, and the other signal representative ofthe value of the cosine wave at the instant of occurrence of the pulsefrom the logic circuit. As will be seen, these two signals represent thenecessary X and Y coordinate velocities in order to maintain the tracinghead over the pattern.

These X and Y coordinate signals are applied through the contacts ofmode relay 21 and speed control rheostat 22 to the traverse controlcircuit 23. A pair of reference voltages of equal positive and negativevalue are also produced inthe sample-and-hold circuit and may besubstituted for the X and Y coordinate signals in which case thesesignals are then applied to the traverse control circuit 23. The X and Ycoordinate signals are then passed through the traverse control circuitand on to their respective servo amplifiers, X servo amplifier 24 and Yservo amplifier 25. The outputs from these servo amplifiers are appliedto a pair of power amplifiers, the x-power amplifier being generallydesignated 27 and the y-power amplifier being generally designated 28.

The output from the amplifiers is applied through relays 29 and 30 tothe X motor 31 and the Y motor 32. Each of these motors has on its shafta tachometer generator, the X tachometer being designated 33 and the Ytachometer being designated 34. The outputs from these tachometergenerators are fed back to their respective servo amplifiers through thetraverse control.

The necessary DC potentials for operating the electronic circuitry arederived from power supply 35 which is provided with alternating currentsupply, for example, 115 volts 60 cycles through the contacts of relay36. i a

The tracer is controlled by the operator from a control panel shown inthe dotted outline in the lower por tion of FIG. 1 and designated 37containing the control switches for controlling the various functions.For example, control switch 38 controls the operation of relay 36through which primary power is applied to the equipment. Control switch39 controls relays 29 and 30 through which the power amplifiers areconnected to their coordinate drive motors. Switch 40 controls the lightrelay 19 which selectively energizes either the flood lamps 6 or theindicator lights 64. Switch 41 controls the direction of rotation ofmotor 11 and also the actuation of the direction relay 16 which, aspreviously previously indicated, may control the phase of the sine wavesapplied to the preamplifier. Switch 42 controls the priority of the fourposition direction switch 26 to permit straight line cutting without apattern or altematively automatic line acquisition. In its reversedirection it is a momentary contact switch to permit jogging of theapparatus to assist in line acquisition. Switch 43 controls the value ofthe speed signal applied to the direction switch 26 and also controlsthe mode relay 21 to open the tachometer circuits during high speedoperation. Associated with the various switches are, when applicable,various lamps which indicate the condition of the switch or the systemfor the information of the operator.

Considering now FlG. 2, this shows an elevation of the tracing headincluding the elementsshown in the dotted outline to the left of FIG. Iand bearing the general designation 45. As will be seen, the floodlights 6 provide illumination to the pattern 7 and the light from hepattern 7 is received by lens 8 and directed towards the mirror 9, andback on to the photocell 10. The photocell 10 is mounted directlyv overthe center of the lens 8 and here, while it slightly'reduces theaperture of the lens, it has no effect on the focus of the image fromthe pattern 7. The mirror 9 is mounted in a mirror carrier 46 mounted inthe end of a quill 47 which is fixed to the shaft of motor 11 by meansof a set screw 48. Also mounted within the quill is the permanent magnet12 and centered about the quill are the stator windings 13 and 14 woundon suitable laminated stators 49 and 50.

It will be noted that these stators together with the motor 11 aremounted in a sub-frame which may be rotated in the main housing 52. Acap 53 covers motor 11 and is fixed to the sub-frame 51. Suitablegraduations and indicia are provided at 54 to indicate the relativerotational position of the sub-frame 51 and the main enclosure 52. Thenecessary electrical connections from the motor 11, the stators l3 and14, the photocell 10, the lamps 6 are provided through an electricalconnector 55. It will be noted that the electrical connections to thesub-frame 51 pass through a disconnectable plug and socket 56 so thatthe sub-frame may be completely removed from'the general enclosure asdesired. One reason for making the sub-frame removable is to enable theoperator to replace the mirror assembly mounting 46. it will be seenthat the mirror 9 is offset and at an angle to the axis of rotation ofmotor 11. This angular relationship produces the desired circular scanand the degree of angularity determines the diameter of scan. Bysubstituting various mirror assemblies, the operator can select thedesired scan diameter for reasons previously indicated.

Considering now FIG. 3, it will be seen that the various controls andindicator lights from the control unit are arranged in a single panelfor the convenience of the operator, and include the lights switch 40,the tracer switch 41, the speed control rheostat 22, the power and driveswitches 38 and 39, the high speed push-button 43, the direction controlswitch 26 and the traverse and trace control switch 42. Also includedare indicator lights, a first light 60 to indicate that power is on, asecond light 61 to indicate that the drive is on, a third light 62 toindicate when the machine is in the traverse mode, and a fourth light 63to indicate when the machine is in receipt of a signal indicative of apattern, that is, when the on-pattern relay is energized.

OPERATION The operation of this system will now be described inassociation with the foregoing figures, and in addition, FIGS. 4A to 4Band 5.

To start up the apparatus, the operator turns on power switch 38 thusenergizing the relay 36 which provides the necessary supply voltage tothe power supply. He then switches on drive switch 39 which energizesrelays 29 and 30 thus enabling the motors 31 and 32 to be driven bytheir related power amplifiers. At this point, the indicator lights 64are energized, these lights are not shown in FIG. 2 since they are outof the plane of the section, but are arranged around the lower part ofthe tracing head so as to project a circle of light immediately underlens 8 which approximately defines thetracing circle. The operator maynow put the switch 42 in the traverse position thus illuminatingtraverse light 62 and providing a potential to the contacts of directionswitch 26. By manipulation of direction switch 26 any one or two of thecontacts may be closed, thus causing the various relays in the traversecontrol shown in FIG. 4D to be operated. For example if the directionswitch is moved in the +Y direction the relay 70 is energized, openingthe normally closed contacts 70-1 and closing the normally open contacts70-2, thus applying the potential from terminal74, which is the +speedsignal which has been derived from the sample-and-hold circuit, throughthe contacts of mode relay 21 and the speed control 22 to the traverseunit 23, thus to terminal 74, through the normally closed contacts 71-1,72-1 and 73-1 to the ysignal terminal 75. In a similar way, operation ofthe X contact in the direction switch 26 will actuate relay 71 openingnormally closed relay contact 71-3 and closing normally open contact71-4 thus applying the potential from terminal 74 through contact 71-4,72-3, 73-3 to terminal 76. The Y contact of the direction switchoperates relay 72 and the -X contact operates relay 73 both of whichfunction to provide negative signals to the same terminals 75 and 76from the negative supply on terminal 77 which is derived from thesample-andhold circuit in a manner similar to the potential supplied onterminal 74.

It will be noted that actuation of relay 70 or 72 opens circuits contact70-6 or 72-6, and closes contact 70-5 or 72-5. This provides a returnpath for the drive relays 29 and 30 to ground without going through thecontacts of the on-pattern relay. In a similar manner, actuation ofrelay 71 and 73 provides a return path for relay 29 to ground withoutgoing through the on-pattern relay.

The operation of the direction. switch, therefore, has applied suitablevoltages to terminals 75 and 76 which are then applied to the servoamplifiers 24 and 25, and through the power amplifiers 27 and 28 andrelays 29 and 30 to the drive motors 31, 32 causing these motors torotate with a speed determined by the voltage at terminals 74 and 77,and with a direction determined by the selection of the relay or thedirection of closure of direction switch 26. In this way, the operatorcan maneuver the device until the spot of light produced by theindicator lamps 64 is over the pattern. At this time, the operator mayclose the tracer switch 41 in a direction to provide the direction oftracing desired as shown by the arrow on the control unit causing thetracer to trace clockwise or counterclockwise around the outside of thepattern. I

The closing of tracer switch 41 energizes the motor 11 in one directionor the other, causing it to rotate either clockwise or counterclockwiseand also, may or may not energize relay 16 which operates as a reversingswitch to reverse the outputs applied from the sine and cosinegenerators 13 and 14 before applying them to the preamplifier 15.

Assuming that the operator has located the tracing head over the patternby means of the direction switch and the motor is rotating in a suitabledirection, the mirror is rotated thereby and causes the illuminationfrom a portion of the paper including the pattern to be reflected on tothe photocell 10. As the photocell 10 crosses a transition between alight and a dark area, a signal is produced and applied to thepreamplifier. If the pattern being traced is a line, the signal producedby the photo transistor 10 will be of the general form shown at a inFIG. 5. This signal is applied to terminal of the circuit shown in FIG.4A and through a suitable shaping network to operational amplifier 81which is provided with suitable potentials as shown, derived from thepower supply, and the output from the amplifier appearsat terminal 82 asshown at b in FIG. 5. This output is also supplied to the circuitcomprising transistors 83 and 84, which are arranged as peak-topeakdetectors so that the output from the emitters of these two transistorsrepresents the peak-to-peak value of the signal.

This value, which is a substantially constant voltage which variesonlywith variations in illumination or contrast of the pattern, isapplied to operational amplifier 85 which is so arranged that the outputfrom the amplifier, which appears at terminal 86, is maintained at avalue representative of the median value of the peakto-peak signal. ThisDC signal at terminal 86 is fed back to amplifier 81 to restore the DClevel of the signal and maintain it such that deviations are equal abouta reference zero-axis.

The outputs from the sine and cosine generators shown at c and d in FIG.5 are also applied to the preamplifier through the contacts of relay 16,as previously indicated. These sinusoidal waves are applied tooperational amplifiers 87 and 88 which are arranged to stabilize thevalue of the sine waves. The output from operation amplifier 87 isapplied to a buffer amplifier consisting of transistors 89 and 90, andthe output applied to terminal 9. In a similar manner, the output fromoperational amplifier 88 is applied to buffer amplifier consisting oftransistors 92 and 93 and appears on terminal 94.

The level signal from terminal 86 and the photocell signal from terminal82 are applied to the logic circuit 17, which is shown in greater detailin FIG. 4b. As will be seen, these signals are applied to differentialcomparator 97 which is so arranged that if a signal on terminal 82exceeds the DC level on terminal 86 by a predetermined amount, anegative voltage step is produced as shown at e in FIG. 5. This steppersists as long as the signal at b exceeds the predetermined value asindicated by the dotted line. This voltage is coupled through capacitor98 to the monostable circuit consisting of transistors 99 and 100.

This monostable circuit is so arranged that transistor 99 is normallyconducting and transistor 100 is normally cut off. On receipt of thenegative impulse through capacitor 98, transistor 99 is switched off,thus switching on transistor 100. The monostable circuit 7 y thenreturns to its normal stable condition in a time determined by he valueof resistor 150 and capacitor 151, producing an output of the form shownat f in FIG. on terminal 101. I

1 An impulse is also coupled from the collector of transistor 99 throughcapacitor '102- to a second monostable circuit consisting of transistors103 and 104. Transistor 103 is normally switched on, but the trailingedge of the pulse on the collector of transistor 99 being a negativegoing front, switches off transistor 103, thus'switching on transistor104 by virtueof the coupling through the buffer amplifier consisting oftransistors 105 and 106 which are coupled to the base of transistor 104.This circuit remains in this condition withtransistor 103 out off for atime determined by the value of capacitor 152 and resistor 153primarily. The resultant pulse is applied to the base of transistor I07,and also to thebase of transistor I08 and has a form as shown mg in FIG.5. I

For the sakeof clarity there is shown at h in FIG. 5 a schematic view ofa pattern consisting of a line, shown shaded, a scanning circle, havinga radius designated lead, and the' effective viewing area of the phototransistor designated aperture. As the aperture passes over the patternit produces the impulse shown at a in FIG. 5, which in turn gives riseto the pulse shown at g in FIG. 5. This pulse has a duration asindicated at h in FIG. 5 as the inhibit period and equal toapproximately the time of 516th of one revolution of the scan. Theperiod between the end of the inhibit period and commencement of thenext inhibit period is referred to as a window, and it is only duringthe window period that signals from the photo transistor can producesample output pulses for reasons as will now appear.

As long as transistor 103 is cut off, transistor 107 is turned on andprevents transistor 100 from being turned on even though transistor 99receives an output from a comparitor. However, as soon as the inhibitperiod ends and transistor 103 once more becomes conducting, transistor107 becomes non-conductive and when transistor 99 receives the nextcomparitor pulse and is cut off, transistor I00 becomes-conductivechanging the circuit to its. unstable condition and producing a samplepulse output at terminal 101.

In this way, signals occurring at times other than the preferredtime,'that is, during the window, which is equivalent to saying atperiods other than the inhibit period, cannot produce a sample pulse,because transistor 100 cannot be turned on, and transistor 103 cannothave any effect on the inhibit period because it is already switched offand only negative pulses can be applied to its base from the collectorof transistor 99.

The pulses as shown at g in FIG. '5 are also applied to the base oftransistor 108 as was previously indicated. This transistor togetherwith transistor 109 operate as a darlington pair and produce a potentialon capacitor 110. This circuit is arranged so that a potential of asuitable level to maintain on-pattern relay l8 energized is developed aslong as pulses are being received on the base of transistor 107.Capacitor 110 provides a suitable storage device to ensure that therelay holds in through the intervening periods and permits a slightdelay in acquisition of the pattern. This output appears at terminal111, and is applied through the contacts of relay 19 to the on-patternrelay 18.

The pulse output from terminal 101 and the sine and cosine output fromthe preamplifier terminals 91 and 94 are all applied to thesample-and-hold circuit 20, shown in more detail in FIG. 4c. The pulseson terminal 101 are applied to a shaping circuit including transistors112 and 113, which produce, from the pulse, a suitable sampling pulsehaving a duration of 60 microseconds. This sample pulse is applied tothe gates of a pair of FET transistors I14 and 115. To the input oftransistor 115 is applied a sine wave from terminal 91. To the input oftransistor 114 is applied the output from the differential amplifier. Ifthere is any difference in these two signals during the gating period,it is'amplified by the amplifier 116 and the output from the amplifieradjusted to maintain the voltage at the last sample level. This value isthen applied to operational amplifier 117 and through the bufferamplifier consisting of amplifiers 118 and 119, to the output terminal120. In a similar manner, the sampled value of the cosine wave appearingat terminal 94 is stored in the sample-and-hold circuit and appears atterminal 121.

As will be seen from FIG. 4d already considered, the potentials onterminals 120 and 121 will be applied to the X signal terminal 76 andthe Y signal terminal 75 in the absence of operation of any of therealsy 70, 71,72 of 73. Considering the signal on terminal 76, this isapplied to servo amplifier 24. A further signal from the tacho generator33 is applied through the traverse circuit 23 from terminal 22 (FIG. 4b)through the contacts of relay 123 to the terminal 124 and thence, to theservo amplifier 24. As will be seen in. FIG. 4e, these signals areapplied to an operational amplifier 125 with suitable feedback andcorrelation of the signals to provide a maximum gain with adequatestability. The DC signal thus produced is applied to a buffer amplifierconsisting of transistors 127 and 128 and transistors 129, 130, 131 toprovide a suitable drive signal for power amplifier '27 which provides areversible DC supply to motor 31.

In a similar manner power amplifier 28 provides a reversible DC supplyto motor 32, as has been explained. The amplitude and polarity of the DCsignal applied to the X and Y motors 31 and 32 respectively is afunction of the value of the sinusoid at the instant of occurrence ofthe sampling pulse as illustrated in FIG. 5. The speed of rotation ofthe X and Y motors therefore is proportional to the direction of theline eitpressed as co-ordinate values on an X and Y co-ordinate system.If the motors are now coupled to a suitable drive mechanism they willcause the machine and the tracing head associated therewith to followthe line with a velocity determined by the speed setting. If during itsnormal operation the tracing head ceases to observe a pattern, pulseswill cease to be fed to transistors 108 and 109 and the on-pattern relay18 will open. The opening of relay 18 will cut-off the drive because thereturn path for relays 29 and 30 is through the contacts of relay 18. Asthe same time the on-pattern light will be extinguished because itscircuit is also completed through the contacts of relay 1'8.

At any time when switch 42 is switched in the traverse direction theoperator may control the operation of the machine by means of thedirection switch 26. Therefore at this point if the machine has stoppedbecause it has gone off pattern it is possible for the operator to movethe switch 42 to the traverse position and push switch 26 in a directionsuitable to move the tracing head once more over the pattern. When thepattern has been acquired switch 42 may be returned to the centerposition. At any time during the operation of switch 26 the high speedpushbutton may be pushed. This increases the voltage applied to thespeed circuits and permits the machine to traverse at high speed byactuating relay 123 and opening the tachometer feedback circuit.

Other facilities are also available to the operator which have beenreferred to earlier. In particular, as has been indicated, the diameterof the tracing circle is related to the desired tracing speed andvarying conditions will dictate varying diameters of tracing circle. Tothis end, the machine may be supplied with aplurality of mirror carriers46, with different mirror angles to permit different diameters ofscanning circles. 'Io substitute one for the other is only necessary forthe operator to remove the sub-frame 51, remove the mirror carrier 46and replace with the desired mirror carrier.

It is also possible that under varying conditions the degree of offsetnecessary to compensate for kerf will vary and this is also subject toadjustment by the operator. i

As was previously indicated the sub-frame 51 may be rotated and thedegree of rotation indicated on the indicia 54 will indicate the degreeof correction.

The manner in which rotation of this assembly allows for kerf may beunderstood if one considers that the circular scanning of thetracinghead commences at zero degrees and the system attempts to maintain thecrossing of the pattern coincident with the zero degree point on thecircular scan. If now a radius joining the center of rotation of thetracing head and thezero degree point of the scan is parallel to theline being traced, then the center of rotation of the scan travels alongthe line. If on the other hand, this radius is not parallel to the linebeing traced, then the center of the scan is displaced from the pattern.The machine tool controlled, however, follows the center of the scan dueto direct mechanical coupling and hence does not fol low the pattern butfollows a point displaced from the pattern by an amount depending uponthe angle of the radius with respect to the pattern. This angle is theangle indicated by the indicia 54, and hence the indicia may becalibrated in terms of actual displacement or kerf correction.

SUPPLEMENTARY DISCLOSURE The apparatus above described may be modifiedin various ways to fit particular situations. For example, specialprovision may be made to improve stability of the apparatus when thetracing head unit is remote from the amplifier. In addition, theoperator may be provided with a manual steering control. Theseparticular modifications are described in greater detail in associationwith FIGS. 4e and 4f.

Considering 4e, it will be seen that the phototransistor rather thanbeing connected directly into the input of preamplifier is insteadconnected to an FET arranged as an amplifier incorporated in the tracinghead unit. potential FET 151 is supplied with suitable potential fromterminals M and L and its base is connected to the output of thephoto-transistor 10. The

will be seen that the input and feedback circuit on this amplifierdiffers somewhat from those shown in association with amplifier 81 inFIG. 4a. It will also be noted that there is no provision for a leveloutput. Transistors 83 and 84 are not used in this circuit but ratherzenor diode 152 is applied to the input terminal of amplifier 97. Thiseliminates the necessity for amplifier 85 in FIG. 4a. Thezenorestablishes a constant level at terminal 2 of amplifier 97 which issufficiently accurate for the purpose.

Turning now to FIG. 4f there is shown a circuit for providing a manualsteering control for the operator. This figure should be read withparticular reference to FIG. 1b since the elements shown, in particularthe sample and hold circuit 20 and the traverse circuit 23 occur in bothcircuits and the new elements of FIG. 4f are primarily introducedbetween these two parts of the circuit of FIG. lb.

The connectionsfrom the sample and hold circuit to mode relay 21 are thesame as in FIG. 4b with the same connections to speed control 22.However, the output from the mode relay contacts rather than goingsolely to traverse circuit 23 are applied to an auxiliary circuit. Thisauxiliary circuit includes sine cosine potentiometer which is suppliedwith positive and negative speed signals through transistors 161'and 162which in turn are provided with speed signals from the contacts of moderelay 21, which are applied to the bases of these transistors. Theoutput from the sine cosine potentiometer is applied to the contacts ofthe steer relay 163 and may be used to replace the output from the moderelay whether it be a plus or minus speed signal or a y or x outputsignal. Further contacts on the steer relay will override the on patterncontacts and ensure that terminals T and W of traverse circuit 23 aregrounded when steer relay is energized.

Contacts of switch 42' replace the corresponding contacts in switch 42in control unit 37. It will be noted, for example, that the right handcontact is connected to terminal I of connector J1 on control unit 37.When switched to the traverse position, the mode relay is energized.Switch 164 enables the operator to select either steer or straight cuttraverse. In its left hand position, it energizes the steer relay 163when the mode relay is energized. When in its right hand position itgrounds the contacts of the direction switch 26 in the control unit 37.As seen in FIG. 1b terminal X from switch 164 is connected to terminal Xon the direction switch in control unit 137. High speed switch 165replaces high speed switch 43 in FIG. 1b.

OPERATION Since there is no mechanically rotating element in the tracinghead, it is impossible for the operator to manually steer the tracinghead without this auxiliary control. To this end when the mode relay isactuated and the steer relay is actuated, plus and minus speed signalsare applied to speed control 22 and the output from the speed control isapplied to the contacts of mode relay and from thence to the bases oftransistors I61 and 162. The resultant potential is applied across thesine cosine potentiometer 160. The operator positions the arrow on thesine cosine potentiometer control in a direction corresponding with thatdirection which he wishes the apparatus to move. This locates thesliders on the sine cosine potentiometer in such a manner as to providesignals on the two sliders representative of the necessary co-ordinatesignals to cause the motors to drive the apparatus in a desireddirection. These signals are applied to the contacts of the steer relaywhich is energized and therefore applies these steering signals toterminals K and V of traverse circuit 23. These signals are appliedthrough the system to the X and Y drive motors as previously described.in this way, the operator is free to control the direction of motion ofthe machine-by rotation of the knob of the sine cosine potentiometer andits speed by manipulation of the speed control.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A photo electric edge tracing apparatus including an optical scanningsystem arranged to scan a circular path on a surface bearing anoptically detectable image to be traced, said optical system comprisinga housing,

a subframe rotatably and removably mounted in the upper end of saidhousing, an electric motor mounted in said subframe, a generatorarranged coaxially with said motor with two rotors fixed to the shaft ofsaid motor and at 90 electrical degrees with respect to each other, amirror assembly mounted on the lower end of said shaft, a lens mountedin the lower end of said housing coaxial with said motor, a photocellmounted adjacent the upper surface at the center of said lens and facingsaid mirror assembly, a mirror mounted on said mirror assembly'eccentricwith respect to the axis of said motor with the plane of said mirroralmost at right angles to said axis, said lens and said mirror arrangedto focus a portion of said detectable image on said photocell wherebyrotation of said mirror assembly by said motor causes the photocell toefiectively scan said surface in a circular path.

2. A photo electric edge tracing apparatus as claimed in claim 12wherein the values of the outputs from said generator is determined atthe point in time that the photocell scans the edge being traced andvoltages proportional to said values are used to control motors forpositioning said housing relative to said surface.

1. A photo electric edge tracing apparatus including an optical scanningsystem arranged to scan a circular path on a surface bearing anoptically detectable image to be traced, said optical system comprisinga housing, a subframe rotatably and removably mounted in the upper endof said housing, an electric motor mounted in said subframe, a generatorarranged coaxially with said motor with two rotors fixed to the shaft ofsaid motor and at 90 electrical degrees with respect to each other, amirror assembly mounted on the lower end of said shaft, a lens mountedin the lower end of said housing coaxial with said motor, a photocellmounted adjacent the upper surface at the center of said lens and facingsaid mirror assembly, a mirror mounted on said mirror assembly eccentricwith respect to the axis of said motor with the plane of said mirroralmost at right angles to said axis, said lens and said mirror arrangedto focus a portion of said detectable image on said photocell wherebyrotation of said mirror assembly by said motor causes the photocell toeffectively scan said surface in a circular path.
 2. A photo electricedge tracing apparatus as claimed in claim 12 wherein the values of theoutputs from said generator is determined at the point in time that thephotocell scans the edge being traced and voltages proportional to saidvalues are used to control motors for positioning said housing relativeto said surface.